Low density plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition to process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent to the target impact the surface of the target to dislodge, i.e., "sputter" material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices having trenches or holes with a high depth to width aspect ratio, can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate to position vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered in a low density plasma often has an ionization degree of less than 1% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term "dense plasma" is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in a steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
In a number of deposition chambers such as a physical vapor deposition chamber, the chamber walls are often formed of a conductive metal such as stainless steel. Because of the conductivity of the chamber walls, it is often necessary to place the antenna coils or electrodes within the chamber itself because the conducting chamber walls would block or substantially attenuate the electromagnetic energy radiating from the antenna. As a result, the coil and its supporting structures are directly exposed to the deposition flux and energetic plasma particles. This is a potential source of contamination of the film deposited on the wafer, and is undesirable.
To protect the coils, shields made from nonconducting materials, such as ceramics, can be placed in front of the coil. However, many deposition processes involve deposition of conductive materials such as aluminum on the electronic device being fabricated. Because the conductive material will coat the ceramic shield, it will soon become conducting, thus again substantially attenuating penetration of electromagnetic radiation into the plasma.
The generation of unwanted particulate matter can also be reduced by using a conductive metal shield as a coil as disclosed in copending application Ser. No. 08/730,722, filed Oct. 8, 1996 (pending Attorney Docket No. 1207/PVD/DV) entitled "Active Shield for Generating a Plasma for Sputtering" by Sergio Edelstein and Mani Subramani, which is assigned to the assignee of the present application and is incorporated herein by reference in its entirety. The conductive coil-shield is coupled to an RF source such that the coil-shield itself inductively couples electromagnetic energy to a plasma. Such an arrangement is believed to avoid attenuation of the RF power while at the same time substantially reducing the generation of contaminating particles from the coil-shield.
However, coil-shield designs as described in the aforementioned application have one or more channels in the wall of the coil-shield to separate portions of the coil-shield wall into one or more individual windings. However, sputtered material can pass through the coil-shield channels. To prevent such sputtered material from contaminating the vacuum chamber of the apparatus, it has been proposed to provide another conductive metal shield wall spaced behind the coil-shield channels. However, such an additional shield wall can increase the size of the chamber. In clean room environment where space is at a premium, the chamber should be as compact as possible. Furthermore, a second conductive metal shield wall can cause power losses due to eddy currents in the wall.